A solid state relay provides isolation between a control circuit and a switched circuit and may replace an electromechanical device such as a reed relay. A typical solid state relay consists of a light emitting diode (LED) optically coupled across an electrically isolating gap to a photodiode array. The photodiode array is electrically connected to a output device such as a field effect transistor (FET). Light from the LED creates a voltage across the photodiode array and activates the output FET.
Presently available solid state relays often are susceptible to electrical transients and have relatively slow turn-off characteristics. For example, the relay disclosed in U.S. Pat. No. 4,390,790 to Rodriguez includes a photodiode array directly connected to an output FET. Rodriguez's use of a turnoff transistor to discharge the output FET gate to source capacitance provides some improvement in turn-off speed but provides no transient protection to the relay. Relays such as this are vulnerable not only to transient propagation between the control and switched circuits but also to transient induced false turn-on and turn-off.
In accordance with the illustrated preferred embodiment of the present invention a solid state relay has improved turn-off characteristics and a high degree of transient immunity. The relay includes an optically controlled active series and shunt enhancement circuit located between the photodiode array and a pair of output FETs. Series phototransistors in the enhancement circuit actively couple and decouple the array from the output FETs at turn-on and turn-off. A shunt FET and a shunt silicon controlled rectifier (SCR) create rapid discharge paths for the output FET gate to source capacitances at turn-off. A shunt transistor is capacitively coupled to the relay output to protect against transient induced false turn-ons.
In an alternate preferred embodiment the solid state relay includes a shield over the photodiode array. Use of the shield allows direct and close optical coupling between the LED and the photodiode array so that high speed can be achieved at low required LED currents. The shield is both optically transmissive and electrically conductive and is connected to ground or to another neutral part of the relay circuit. The shield is positioned to intercept control circuit transients so that false turn-on caused by electrical activation of the photodiode array or other components can be avoided.